Al₅BO₉: A Wide Band Gap, Damage‐Tolerant, and Thermal Insulating Lightweight Material for High‐Temperature Applications

Abstract

The electronic structure, chemical bonding, mechanical, and thermal properties of a B‐containing mullite Al5BO9 were investigated using a combination of first‐principles calculations, Debye model, Clack model, and Slack's equation. The results show that Al5BO9 has a wide band gap of 6.4 eV. The bonding in BO3 triangle and AlO4 tetrahedron is covalent‐ionic, whereas that in AlO6 octahedra and AlO5 bipyramids is ionic‐covalent. The maximum (245 GPa) and minimum Young's modulus (161 GPa) are along [100] and [010] direction, respectively. Based on the low shear modulus (78 GPa) and low Pugh's ratio (G/B = 0.497), Al5BO9 is predicted as a damage‐tolerant ceramic. The most likely slip systems are [010](100) and [100](010). The high Debye temperature (956 K) indicates good high‐temperature stability. The minimum thermal conductivity is estimated to be 1.59 W·(m·K)−1, and the thermal conductivity decreases with temperature as 2071.9/T.

Mechanical and thermal properties of light weight boron‐mullite Al5BO9

“…Nevertheless, the experimental results are comparable with the theoretical values. In accordance with the theoretical predicted data, 12 shear modulus of Al 5 BO 9 is much lower than the Young's modulus E and bulk modulus B . Low shear modulus indicates low shear deformation resistance of Al 5 BO 9 .…”

“…Table 4 shows the bulk modulus, shear modulus, Young's modulus, Pugh's ratio, and Vickers hardness of polycrystalline Al 5 BO 9 . The theoretically predicted data of Zhou et al 12 are also included for comparison. It is worth mentioning that the dimensional error of the sample for IET measurement exists.…”

“…Lower density makes Al 5 BO 9 promising in aerospace applications. Previous work of Zhou et al 12 has predicted that Al 5 BO 9 is a wide band gap (6.4 eV) insulator with a minimum thermal conductivity of 1.59 W m ‐1 K −1 . It also has low shear modulus of 78 GPa and a low Pugh's ratio of 0.497, indicating its ability to tolerate damage.…”

“…Fischer et al 10 further extended the concept to borides with mullite octahedral chain characteristics. As a new type of boron mullite, Al 5 BO 9 attracted great interests recently 10,11,12 . The crystal structure of Al 5 BO 9 in Figure 1A shows that there are 60 atoms in the unit cell, which consists of AlO 6 octahedron chains, AlO 4 tetrahedron, and BO 3 triangular structural units.…”

Universal fragment descriptors for predicting properties of inorganic crystals

“…Furthermore, interaction diagrams as depicted in Figure 5 offer a pathway to design materials that meet certain constraints and requirements. For example, substantial differences in thermal expansion coefficients among the ma-terials used in high-power, high-frequency optoelectronic applications leads to bending and cracking of the structure during the growth process [91,102]. Not only would this work-flow facilitate the search for semiconductors with large band gap energies, high Debye temperatures (thermal conductivity), but also materials with similar thermal expansion coefficients.…”